1
$\begingroup$

According to Bernoulli’s principle, airflow speeds up over an airfoil which decreases the pressure, and I read somewhere that when airflow speeds up over an airfoil, this creates a vacuum. Is this statement true?

How would a vacuum above an airfoil be different from airflow separation?

$\endgroup$
  • $\begingroup$ See Chapter 3 here av8n.com/how/#contents. True, there may be some vacuum formed to suck the plane up in the air, but I think that is more than overcom by the air being pushed down by the wing, and conversely the air pushing the wing up in elevation. And of course, the many related topics over on the right. $\endgroup$ – CrossRoads Nov 13 '18 at 16:09
  • $\begingroup$ @CrossRoads Okay now if there is a vacuum over the wing, wouldn’t be the same as airflown separation ? $\endgroup$ – Ismail El-Shaarawy Nov 13 '18 at 16:48
  • $\begingroup$ See 3.7 and Figure 3.11. The low pressure (vacuum) area can be seen to be pretty small: The second type of stall-warning device (used on the Cessna 152, 172, and some others [my 177], not including the 182) operates on a different principle. It is sensitive to suction at the surface rather than flow along the surface. It is positioned just below the leading edge of the wing, as indicated in the right panel of figure 3.11. $\endgroup$ – CrossRoads Nov 13 '18 at 16:56
  • $\begingroup$ At low angles of attack, the leading edge is a low-velocity, high-pressure region; at high angles of attack it becomes a high-velocity, low-pressure region. When the low-pressure region extends far enough down around the leading edge, it will suck air out of the opening. The air flows through a harmonica reed, producing an audible warning. $\endgroup$ – CrossRoads Nov 13 '18 at 16:57
  • 1
    $\begingroup$ I'm just using "partial vacuum" as shorthand for lower pressure in one spot than in an adjacent one. Probably not good as a scientific or engineering term. $\endgroup$ – John K Nov 13 '18 at 19:52
2
$\begingroup$

I think this is simply a terminology issue.

There is no vacuum in the physical sense on the upper surface of an airfoil. There is an area of low pressure, though, which in certain context is also called a vacuum.

A great example for this linguistic imprecision is my vacuum cleaner, inside of which there is no vacuum in its physical sense by far, but clearly it sports considerably lower pressure than the atmosphere in the rest of my living room.

$\endgroup$
  • $\begingroup$ I agree with you because according to "No Motion, No Lift" sub section in the following link, there has to be fluid for Lift to exist. NASA $\endgroup$ – Ismail El-Shaarawy Nov 14 '18 at 10:11
1
$\begingroup$

How much of a vacuum there is depends on flight speed. At subsonic speed the amount by which the static pressure on the top forward part of the airfoil is lowered is only a few percent of the static pressure. However, at hypersonic speed the pressure difference is substantial.

How much pressure is reduced can be seen by the lower effectiveness of conventional vertical surfaces when speed is increased. Part of that is aeroelasticity - the loads deform the structure, making the vertical less effective. But when compared with ventral fins, the pressure difference becomes obvious: The ventral fins deform as well, but their contribution to directional stability becomes dominant at higher Mach numbers because they enjoy the luxury of working in air of higher density.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.